1. Field of the Invention
The present invention relates to an image display apparatus and, more particularly, to a compact head- or face-mounted image display apparatus which is retained on the user's head or face to project an image into his or her eyeball.
2. Background of the Related Art
In recent years, helmet- and goggle-type head- or face-mounted image display apparatuses, which are designed to be retained on the user's head or face, have been developed for the purpose of enabling the user to enjoy virtual reality or a wide-screen image by oneself.
A typical image display apparatus of the type described above is composed of an image display device, e.g., a liquid crystal display device (LCD), and an optical system for projecting an image formed by the image display device into an observer's eyeball. There are two ways of widening the view angle of the image display apparatus: one in which the focal length of the optical system is shortened; and another in which the size of the image display device is increased.
However, if the size of the image display device is increased to widen the view angle, the overall size of the image display apparatus increases, so that the load imposed on the user when wearing the apparatus on his or her head increases. Thus, this method is unfavorable from the viewpoint of practical use. Accordingly, to widen the view angle by using a compact optical system, the power of the entire optical system must be increased.
FIG. 12 shows one example of conventional projection optical systems [e.g., Japanese Patent Application Laid-Open (KOKAI) No. 3-191389 (1991)]. The optical system includes an LCD 1 for displaying an image, and a half-mirror surface 2 which is disposed at a tilt on the intersection point of the optical axis of the LCD 1 and an observer's visual axis to lead a bundle of rays from the image formed by the LCD 1 to an observer's eyeball E. The optical system further includes a concave mirror 3 of positive power which is disposed to face the LCD 1 across the half-mirror surface 2. The half-mirror surface 2 and the concave mirror 3 are integrated into a prism 4. This type of optical system is known to be capable of displaying an image of good quality with a compact structure.
To widen the viewing angle of the above-described optical system, it is necessary to increase the power of the optical system, as described above. One method of increasing the power of the optical system is to reduce the curvature of the concave mirror 3. It is also possible to adopt a method wherein, as shown in FIG. 13, a lens 5 of positive power is disposed between the eyeball E and the half-mirror surface 2 [see Japanese Patent Application No. 5-35212 (1993)].
However, as the power of the concave mirror 3 or the power of the lens 5, which is disposed between the eyeball E and the half-mirror surface 2, is increased, the object surface where the LCD 1 should be disposed comes closer to the concave mirror 3. Eventually, it becomes impossible to dispose the LCD 1 because of interference with the prism 4.
Thus, when an LCD having a diagonal of about 1.3 inches and a height-to-width ratio of 3:4 is used as the image display device 1, the horizontal viewing angle of 45.degree. is the widest angle attainable for the above-described optical system by the conventional technique of widening the viewing angle.